High temperature cobalt-base alloy



United States Patent HIGH TEMPERATUltE COBALT-BASE ALLOY John C. Freche,Fairview Park, Stanley J. Klima, Rocky River, and Richard L. Ashbrook,Berea, Ohio, assignors to the United States of America as represented bythe National Aeronautics and Space Administration No Drawing. Filed Mar.26, 1964, Ser. No. 355,126

4 Claims. (Cl. 75170) The invention described herein may be manufacturedand used by or for the Government of the United States of America forgovernmental purposes without the payment of any royalties thereon ortherefor.

The present invention relates to an improved cobalt-base alloy having ahigh load carrying capacity at elevated temperatures. The invention isfurther concerned with a cobalt-base alloy that is stable in a highvacuum and exhibits good corrosion resistance characteristics.

Present day aerospace structures require alloys which can be subjectedto thedual environment of liquid metals on one surface and a high vacumon the other. Examples of such structures are components forturbo-electric space power systems in which nuclear power is convertedto electric power through the medium of a closed thermodynamic cycle.Such systems have many components, such as reactor, radiator, ductingand various turbo-generator parts, and the ducting as well as theradiator components present some extremely critical materials problems.For example, the material in these components, must be ductile tofacilitate forming as well as corrosion resistant and have good hightemperature strength properties. Pump and turbine components representother examples of aerospace structures requiring improved hightemperature strength and ductility.

Certain stainless steels as well as wrought nickel and cobalt-basealloys have been considered for ducting in turbo-electric space powersystems. Also refractory metal alloys of columbium have been consideredfor temperatures of 2000 F. and above.

Each of these types of materials has certain limitations when consideredfor use in turbo-electric space power system ducting applications. Forexample, stainless steels are limited to approximately 1400 F. for longlife, even at the relatively low stress levels likely to be encounteredin ducting for turboelectric space power systems. Wrought commercialcobalt and nickel-base alloys are limited to approximately 1600 and 1700F., respectively, for long life at low stress levels. Some metals tendto evaporate more than others in a high vacuum because the vaporpressures of the various metals differ, and chromium as well as aluminumare particularly susceptible to evaporation losses. Virtually allstainless steels, cobalt-, and nickel-base alloys contain appreciablequantities of chromium, and most nickel-base super alloys containaluminum, as well. Therefore, evaporation of these elements may occurduring long time exposure of these alloys to a high vacuum environment.As a consequence, the structural integrity of these alloys may beaffected. The manner in which the alloying element is tied up in themetal matrix can greatly alfect this process. By way of example, a solidsolution of chromium in a metal matrix would be very likely to be morereadily affected by evaporation than chromium that is part of aninter-metallic minor phase.

A solution to the problem would be the elimination of high vaporpressure alloying elementsin the alloys. From a corrosion resistancestandpoint, cobalt resists corrosion by mercury more than nickel butless than iron. It appears that cobalt is at least equivalent to nickelin corrosion resistance in alkali metals up to the limit of its usefultemperature range. Certain stainless steels, though acceptable up to1600 F. in contact with the alkali metals,

show a low compatibility with mercury if they have a high nickel and/ora high chromium content. Nickelbase alloys are not compatible withmercury, but may be used with the alkali metals up to approximately 1700F. Refractory colurnbium alloys, although having excellent elevatedtemperature strength characteristics and corrosion resistance to bothmercury and the alkali metals, are very subject to oxidation. This makespilot or ground tests of prototype units using this material extremelydifficult and expensive.

It is, therefore, an object of the present invention to provide acobalt-ba-se alloy capable of high load-carrying capacity attemperatures up to 1850 P. so that the increases in efficiency possiblethrough operation at high cycle temperatures may be realized withadvanced turboelectric space power systems.

Another object of the invention is to providean alloy which hasresistance to the corrosion of contacting heat transfer andturbine-drive-fluid media.

Another object of the invention is to provide an alloy which is stablein the high vacuum of space at elevated temperature.

A further object of the invention is to provide a cobaltbase .al'loythat is workable so that it may be fabricated int-o sheet or tubing forducting and radiator applications.

These and other objects and advantages of the invention will be apparentfrom the specification which follows.

The present invention is embodied in alloys having the followingcomposition range:

The addition of zirconium up to about three percent to the abovepreferred alloy composition further improves the strength properties.For example, 1800 F.-15,000 p.s.i. rupture lives in air in excess of 350hours and 1850 F.l5,000 p.s.i. rupture lives in excess of hours havebeen obtained with addition of zirconium to the preferred composition inwhich the cobalt content is lowered to accommodate the addition. Thus, amore preferred alloy has the following composition:

Percent Cobalt 73.1 Tungsten 25.0 Titanium 1.0 Zirconium 0.5 Carbon 0.4

The subject alloys were prepared with one of the simplest possiblecasting techniques. The melt was made in a refractory crucible ofzirconia which was placed in a high frequency induction coil.

The bottom of a cold zirconia crucible was covered with a small quantityof electrolytic cobalt. On top of this, carbon was placed in the form ofone inch diameter compacts of lamp black. This was covered withbriquetted titanium sponge. The whole was covered with electrolyticcobalt nearly filling the crucible. A cylindrical shield was placedaround the top of the crucible, and a flow of argon was directed at thetop of the charge.

Once the charge had begun to settle, the remaining cobalt was added.When this portion of the charge was completely melted, tungsten wasadded'i'n' th'efornfof short lengths of A2 diameter rod.

The melt was then superheated to approximately 3050 F. and held forthree minutes to insure that the tungsten was melted. The melt was thenallowed to cool to approximately 2900 F. and was poured. During pouringthe inert gas coverage was removed. Melts were hand poured intoinvestment molds heated to 1600 F., and were permitted to come toequilibrium temperature naturally without speeding up the processartificially. These alloys have also been prepared by more complextechniques, such as closely controlled vacuum melting, which resulted infurther improvements in alloy properties. Thus, by introducing a higherdegree of complexity in the casting process, an improved alloy results.

The alloys of this invention derive their high elevated temperaturestrength from the solid solution strengthening of the cobalt bytungsten, by the precipitation of the intermetallic WCo phase, and bythe formation of dispersed tungsten and titanium carbides. Samples ofone of the preferred compositions, Co-25W-1Ti-0.4C, made in accordancewith the simplified casting technique described above provided anaverage ultimate strength of 44,900 p.s.i. and an average elongation of12.5% at 1800 F.

A comparsion of stress rupture properties of the preferred compositionswith some of the strongest cobaltbase alloys commercially available isshown in Table I.

TABLE I Average Alloy Stress, Temp, Rupture psi. F. Life in air, hrs.

Co-25W-1'Ii-0.4C 15, 800 1, 800 90 5, 1,800 200 Co 25W-1'I1-0.5Zr-0.4C000 1 850 90 Wl-52 (Cast). 15,000 1,800 60 IIS-Bl (Cast) 15,000 1, 80010 ITS- (Wrought) 15,000 1, 800 10 The results of capsule corrosiontests of several alloys in mercury are shown in Table II.

From the above information, it is evident that the high strength of thealloys of the present invent-ion is superior to that of commonly usedcobalt-base alloys. Cobaltbase alloys are used for tur-bine vanes andbuckets in jet engines, and these components are generally coated toimprove oxidation resistance to combustion gases. It is evident that theimproved alloys of the present invention are superior for these turbinecomponents when the proper oxidation resistant coatings are used. Inaddition, the combination of high strength and ductility of this alloyseries makes these alloys, when coated, desirable for use in combustionchamber and tailpipe assemblies.

The resistance to evaporation of this alloy series makes 7 it applicableto high temperature vacuum furnace components where loss of strength canbe a serious problem. It could be used as-cast for structural componentsor as sheet for radiation shields. In the latter application, whereloading is not particularly severe, these alloys may be used up to about2200 F. or even higher, and no coatings are required for thisapplication.

It is understood that equivalents or modifications of or substitutionsfor parts of the above described embodiments of the invention may bemade without departing from the spirit of the invention or the scope ofthe subjoined claims.

What is claimed is:

1. A cobalt base alloy capable of high load carrying capacity atelevated temperatures consisting essentially of from 48.5% to 84.5%cobalt, from 15% to 45% tungsten, from 0.4% to 2.5% titanium, and from0.1% to 1.0% carbon.

2. The cobalt base alloy of claim 1 additionally containing up to 3%zirconium, the cobalt content of said alloy being adjusted toaccommodate the addition.

3. A cobalt base alloy capable of high load carrying capacity atelevated temperatures consisting essentially of 73.6% cobalt, 25.0%tungsten, 1.0% titanium, and 0.4% carbon.

4-. A cobalt base alloy capable of high load carrying capacity atelevated temperatures consisting essentially of 73.1% cobalt, 25.0%tungsten, 1% titanium, 0.5% zirconium, and 0.4% carbon.

References Cited by the Examiner UNITED STATES PATENTS 2,829,048 4/1958Cockardt et al. 170 3,091,022 5/1963 Faulkner 75l70 X 3,118,763 1/1964Thielernann 75l70 DAVID L. RECK, Primary Examiner.

R. O. DEAN, Assistant Examiner.

1. A COBALT BASE ALLOY CAPABLE OF HIGH LOAD CARRYING CAPACITY ATELEVATED TEMPERATURES CONSISTING ESSENTIALLY OF FROM 48.5% TO 84.5%COBALT, FROM 15% TO 45% TUNGSTEN, FROM 0.4% TO 2.5% TITANIUM, AND FROM0.1% TO 1.0% CARBON.